diff --git a/compos/tests/java/android/compos/test/ComposTestCase.java b/compos/tests/java/android/compos/test/ComposTestCase.java
index bd011fa..b31f4f3 100644
--- a/compos/tests/java/android/compos/test/ComposTestCase.java
+++ b/compos/tests/java/android/compos/test/ComposTestCase.java
@@ -24,11 +24,13 @@
 import static com.google.common.truth.Truth.assertWithMessage;
 
 import static org.junit.Assume.assumeFalse;
+import static org.junit.Assume.assumeTrue;
 
 import android.platform.test.annotations.RootPermissionTest;
 
 import com.android.microdroid.test.host.CommandRunner;
 import com.android.microdroid.test.host.MicrodroidHostTestCaseBase;
+import com.android.tradefed.device.TestDevice;
 import com.android.tradefed.log.LogUtil.CLog;
 import com.android.tradefed.result.FileInputStreamSource;
 import com.android.tradefed.result.LogDataType;
@@ -85,6 +87,8 @@
         assumeDeviceIsCapable(getDevice());
         // Test takes too long to run on Cuttlefish (b/292824951).
         assumeFalse("Skipping test on Cuttlefish", isCuttlefish());
+        // CompOS requires a protected VM
+        assumeTrue(((TestDevice) getDevice()).supportsMicrodroid(/*protectedVm*/ true));
 
         String value = getDevice().getProperty(SYSTEM_SERVER_COMPILER_FILTER_PROP_NAME);
         if (value == null) {
diff --git a/docs/custom_vm.md b/docs/custom_vm.md
index 5511758..d52aa95 100644
--- a/docs/custom_vm.md
+++ b/docs/custom_vm.md
@@ -194,7 +194,8 @@
     "protected": false,
     "cpu_topology": "match_host",
     "platform_version": "~1.0",
-    "memory_mib" : 8096
+    "memory_mib" : 8096,
+    "console_input_device": "ttyS0"
 }
 ```
 
@@ -296,4 +297,16 @@
 * DNS: 8.8.8.8 (or any DNS server you know)
 
 These settings are persistent; stored in chromiumos_test_image.bin. So you
-don’t have to repeat this next time.`
+don’t have to repeat this next time.
+
+### Debugging
+
+To see console log, check
+`/data/data/com.android.virtualization.vmlauncher/files/console.log`
+
+For ChromiumOS, you can ssh-in. Use following commands after network setup.
+
+```shell
+$ adb kill-server ; adb start-server; adb forward tcp:9222 tcp:9222
+$ ssh -oProxyCommand=none -o UserKnownHostsFile=/dev/null root@localhost -p 9222
+```
diff --git a/vmbase/src/uart.rs b/vmbase/src/uart.rs
index 09d747f..e35555d 100644
--- a/vmbase/src/uart.rs
+++ b/vmbase/src/uart.rs
@@ -16,7 +16,6 @@
 //! provided by crosvm, and won't work with real hardware.
 
 use core::fmt::{self, Write};
-use core::ptr::write_volatile;
 
 /// Minimal driver for an 8250 UART. This only implements enough to work with the emulated 8250
 /// provided by crosvm, and won't work with real hardware.
@@ -41,7 +40,11 @@
         // SAFETY: We know that the base address points to the control registers of a UART device
         // which is appropriately mapped.
         unsafe {
-            write_volatile(self.base_address, byte);
+            core::arch::asm!(
+                "strb {value:w}, [{ptr}]",
+                value = in(reg) byte,
+                ptr = in(reg) self.base_address,
+            );
         }
     }
 }